1. Field of the Disclosure
The present invention relates generally to power supplies. More specifically, examples of the present invention are related to lighting systems including dimming circuitry for use with power supplies.
2. Background
Electronic devices use power to operate. Power is generally delivered through a wall socket as high voltage alternating current (ac). A device typically referred to as a power converter or as a power converter can be utilized in lighting systems to convert the high voltage ac input into a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used due to their high efficiency, small size, and low weight to power many of today's electronics. During operation, a switch included in a driver circuit of the power converter is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency or varying the number of pulses per unit time of the switch in a power converter.
In one type of dimming for lighting applications, a TRIAC dimmer circuit, or a thyristor dimmer circuit, removes a portion of the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit removes. As such, removal of half the ac input voltage in a half line cycle by a TRIAC dimmer circuit corresponds to a phase angle of 90 degrees. In another example, removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees.
Although phase angle dimming works well with incandescent lamps that receive the altered ac line voltage directly, phase angle dimming typically creates problems for light emitting diode (LED) lamps driven by a switched mode power converter. Conventional regulated switched mode power converters are typically designed to ignore distortions of the ac input voltage and deliver a constant regulated output until a low input voltage causes them to shut off. As such, conventional regulated switched mode power converters cannot dim LED lamps. Unless a power converter for an LED lamp is specially designed to recognize and respond to the voltage from a TRIAC dimmer circuit in a desirable way, the dimmer circuit can produce unacceptable results such as flickering of the LED lamp.
Another difficulty in using TRIAC dimmer circuits with LED lamps comes from a characteristic of the dimmer circuit itself. For instance, a TRIAC dimmer circuit is a semiconductor component that behaves as a controlled ac switch. In other words, it behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps use more than enough current from the ac power source to allow reliable and consistent operation of a TRIAC dimmer circuit. However, the low current used by efficient power converters to drive LED lamps may not draw sufficient current to keep the dimmer circuit conducting for the expected portion of the ac line period.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.